Mr. Camell, who is homeless, was hiding from the 11-degree weather in the WellSprings Center Friday afternoon, enjoying a cup of coffee. He planned to stick around until dinner was served, then head to a public library until 8 p.m., when another shelter opens its doors for the night.

With Friday's high at 14 degrees, and a string of similarly shivery days in the forecast, Pittsburgh residents would do well to prepare like Mr. Camell. More than a week of below-average temperatures loom ahead, including a high of 9 degrees on Wednesday, according to the National Weather Service. On Tuesday, the high will be a cruel 4 degrees.

If temperatures for the rest of the month follow the current forecast, this month will have an average high of 20.8 degrees, making it the coldest January since 1981, said Brad Rehak, a meteorologist at the National Weather Service in Pittsburgh. It would be the seventh-coldest January in Pittsburgh history, remaining far behind record-holder 1977, when the average high was a bone-chilling 11.4 degrees.

The unrelenting cold brings a risk of hypothermia, frostbite, dehydration and wind burn. The threat is especially high for children, the elderly, and those with diseases that affect blood vessels, such as diabetes, according to experts at Duquesne University.

Mr. Rehak blames the cold on the polar vortex, a flow of frigid air from the north. This year, a weakness in the cross-country jet stream has allowed the vortex to penetrate into the Midwest and Northeast.

Storm systems known as "Alberta clippers" have also contributed to the recent cold, Mr. Rehak said, riding the jet stream with cold air from the north. Another clipper is expected to hit on Monday, dragging temperatures back down into the single digits after a respite in the 20s on Saturday and Sunday.

If the weather forecast gives you goosebumps, Mr. Rehak has good news: the invasive polar air is set to retreat late next week, bringing temperatures almost back to normal.

Already, the cold's effect on the city has gone beyond rosy noses and cheeks. It has caused delays and cancellations at public schools across the city. It has given headaches to hundreds of Pittsburgh drivers; the local AAA received 35 percent more calls than on a usual winter Friday, mostly due to dead car batteries, said vice president of community relations Bevi Powell.

The cold has also led shelters to change their routines to give the homeless more protection against the cold. Jay Poliziani, director of the Pleasant Valley Shelter for Men, has noticed increased demand during the recent cold spell. The shelter has increased its capacity by letting homeless sleep on its couches and is allowing them to linger later in the morning. It has also handed out gloves, hats and hand-warmers donated by local churches.

"In past winters we've generally been full most nights, but this one we're full every night," Mr. Poliziani said. The shelter has been forced to turn people away, referring them to another shelter.

With chunks of ice agglomerating in the rivers, Cmdr. Lindsay Weaver of the Port of Pittsburgh Commission has issued a safety advisory, telling boats to use extreme caution. Apart from the danger of running into ice, there is a risk that when ice freezes or unfreezes it could affect water levels, causing tow lines to break, said Jim McCarville, executive director at the commission.

"This is probably the worst in almost 20 years that we are anticipating," Mr. McCarville said. "Everyone needs to exercise caution and know it's going to get worse."

For his part, Mr. Camell said he doesn't mind the cold that much. After all, he's well-equipped, and he and his friends have places to go.

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